It is well known that a combination of reactive oxidant species can be beneficial to water treatment, cleaning, decontamination and remediation applications as they will combat a variety of substrate types which may be present and react with a variety of oxidation byproducts during their breakdown.
Hydroxyl Radicals
Of the common oxidants used in water treatment and remediation, the hydroxyl radical has the most positive standard oxidation potential of 2.80 V and is very effective at oxidizing a wide variety of substances. Hydroxyl radicals react very rapidly with a wide variety of oxidizable substrates. However, the hydroxyl radical lifetime is very short in aqueous media, merely several nanoseconds, and therefore must be produced with several tens of angstroms of a target substrate due to minimal diffusion path length. Hydroxyl radicals can further be quenched by undesirable reactions including reactions with radical quenchers, precursor oxidants and other hydroxyl radicals. For example, carbonate and bicarbonate ions present in natural waters are effective radical quenchers. Further, hydrogen peroxide and ozone can react with hydroxyl radicals; therefore while generating hydroxyl radicals from hydrogen peroxide and/or ozone precursors in water, the precursor is traditionally kept below 10 g/mL to avoid excessive consumption of hydroxyl radicals by the parent oxidant.
One issue with using hydroxyl radicals in water treatment is their ability to oxidize halide salts with much lower standard potentials and even oxidize sulfate diaion to the persulfate radical anion. A single electron oxidation of halide by a hydroxyl radical will produce hypochlorous acid, hypobromous acid and their hypohalite forms depending on the pH. However, an excess of hydroxyl radicals in the presence of hypohalites will further oxidize them in subsequent steps to chlorate, which is toxic, and bromate, which is carcinogenic.
Fenton Catalyst Activation
Fenton catalyst activation of hydrogen peroxide occurs when a reduced iron species, Fe2+, is oxidized by hydrogen peroxide thereby producing hydoxyl radical, .OH, and an oxidized iron species, Fe3+. The catalytic cycle is completed when hydrogen peroxide reduces Fe3+ back to Fe2+ thereby producing hydroperoxyl radical HOO., which is in equilibrium with superoxide. The Fenton process is summarized in Equations A and B, below.Fe2++H2O2→Fe3++.OH+OH−  Eq. A:Fe3++H2O2→Fe2++.OOH+H+  Eq. B:Similar Fenton-like chemistry occurs with other peroxides such as peroxyacetic acid. Iron sulfate is the most common Fenton catalyst and must be used at a pH near or below pH 4 to avoid excessive precipitation of Fe3+ oxides and oxyhydroxides. Other iron catalyst forms such as iron minerals (e.g., magnetite) and chelated iron compounds have stability at higher pH.
Ultrasound Activation
Ultrasound activation of hydrogen peroxide in aqueous solution occurs when ultrasound waves induce cavitation of water forming bubbles, which leads to very high localized heating as cavitation bubbles collapse resulting in the thermal dissociation of hydrogen peroxide to hydroxyl radicals in Equation C.H2O2+heat→2.OH  Eq. C:Similar thermal dissociation of peracids occurs to generate two different radical species in Equation D.AcOOH+heat→AcO.+.OH  Eq. D:
Ultraviolet Activation
Ultraviolet light activation of hydrogen peroxide occurs by the absorption of ultraviolet light, typically in the wavelength range of 180 to 220 nanometers, which leads to dissociation of hydrogen peroxide forming hydroxyl radicals summarized in Equation E.H2O2+UV light→2.OH  Eq E:Similar ultraviolet activation and dissociation of peracids occurs to generate two different radical species in Equation F.AcOOH+UV light→AcO.+.OH  Eq. F:
Thermal Activation:
Thermal activation of hydrogen peroxide can be conducted by impinging a liquid, spray, mist, vapor, or steam containing hydrogen peroxide upon a hot surface coated with a catalyst (e.g., silver oxide, iron oxide, ruthenium oxide, glass, quartz, Mo glass, Fe3-xMnxO4 spinels, Fe2O3 with Cu-ferrite, MgO and Al2O3.) and heated to above 200° C., to form hydroxyl radicals in Equation G.H2O2+heat+catalyst surface→2.OH  Eq. G:
The initial peroxide activation step in Equation G is followed by a series of radical propagation steps in the gas phase where intermediate radical species form such as the hydroperoxyl radical.
Singlet oxygen is a molecular oxygen in an excited electronic state. Singlet oxygen is most commonly produce in aqueous solutions by photolysis of dissolved oxygen directly by ultraviolet radiation or indirectly by energy transfer from a visible light photosensitizer dye to molecular oxygen. The use of photosensitizing dyes such as methylene blue, certain metalloporphyrins, semiconductors and other materials to generate singlet oxygen to degrade contaminants in water, disinfection and other uses are not practical for wastewater treatment due to degradation of dyes by singlet oxygen over time (i.e., photobleaching) and at elevated concentrations.
Another common method of singlet oxygen generation is by chemical reactions where singlet oxygen is released as a byproduct, including the Haber-Weiss reaction, reaction between hydrogen peroxide and hypochlorite, decomposition of 9,10-diphenylanthracene endoperoxide and a reaction between neutral and ionized forms of organic peroxyacids. However, these methods cause the rapid quenching of the singlet oxygen species by physical and chemical pathways. Chemical quenching reactions occur when singlet oxygen is consumed by a non-beneficial chemical reaction involving electron transfer. Physical quenching reactions occur by radiative or non-radiative relaxation of the excited state by physical contact with its surroundings without electron transfer. In these methods, excess hydrogen peroxide is a very effective quenching agent resulting in little or no oxidative activity from singlet oxygen generated in the presence of significant concentrations of hydrogen peroxide. When hydrogen peroxide is present in significant concentrations, as is the case for most commercially produced peroxyacetic acid, singlet oxygen is rapidly quenched by hydrogen peroxide, which reduces singlet oxygen concentration. Chlorine, azide, certain tertiary amines and beta-carotene are other known examples of singlet oxygen quenchers.
Peroxyacetic acid (i.e. AcOOH) is typically made by commercial producers by an equilibrium reaction between concentrated acetic acid (i.e. AcOH). The equilibrium reaction can be catalyzed by a mineral acid such as sulfuric acid at a pH<1 and occurs over a time period of several hours to several days depending on the concentration of hydrogen peroxide, acetic acid and acid catalyst. There is typically a significant concentration of residual hydrogen peroxide and acetic acid in peroxyacetic acid made by the equilibrium reaction. For example, the [peroxyacetic acid][H2O]/[acetic acid][H2O2] concentration ratios are often between 1.8 and 2.5 for commercial grades between 5 and 30 wt % peroxyacetic acid. Peroxyacetic acid solutions are generally unstable at room temperature and pose a significant fire hazard. Therefore peroxyacetic acid is typically produced on site by the equilibrium process or shipped in vented containers from a producer. Peroxyacetic acid may be distilled under reduced pressure to obtain a pure form with low hydrogen peroxide residual, however, distillation is generally not practical and can create a severe explosion hazard.
Superoxide is the radical anion form of molecular oxygen and is a mild reducing agent with a standard oxidation potential commonly reported as −0.33 V in aqueous environments. Superoxide can be produced in bulk as the anhydrous potassium salt, KO2, which rapidly reacts with water or carbon dioxide releasing molecular oxygen and potassium hydroxide or potassium carbonate, respectively. Superoxide can also be produced in situ by ultraviolet irradiation of oxygen containing solutions including seawater, enzymatic processes and by electrochemical reduction of oxygen. For large scale applications superoxide is typically supplied as a bulk chemical or generated in situ from activated hydrogen peroxide reactions. Potassium superoxide is a water-sensitive hazardous material and combustion aid, which may be prohibitive barriers to its use in some locations. Also, potassium superoxide must be fed into a treatment process as a solid feed, which can be problematic due to water absorption, caking and clogging of solid feeders.
Several common issues arise with conventional reactive oxygen species formulations including, for example, limited shelf life, low mobility of oxidants and/or catalysts; highly acidic or alkaline oxidants which cause significant changes in the natural soil or groundwater pH; limited options for oxidant types available from a single product or system; and logistic, cost, permitting or safety issues associated with bringing large quantities of strong oxidizers and hazardous chemicals on site. Additionally, the use of conventional iron-based hydrogen peroxide Fenton catalysts and sodium persulfate activators, such as iron (II) sulfate, require an acidic pH of less than 4 to be active, but as the pH increases toward neutral pH levels the precipitation of iron oxides and oxyhydroxides occurs. Precipitated iron can cause pore plugging in soils, fouling and staining equipment and can promote population blooms of iron bacteria which cause biofouling of soils, and accelerated microbial corrosion of steel well casings, pipes and equipment.
Well Flushing:
Oil and gas production wells, groundwater wells and water pipelines are often hyper-chlorinated to control microbial growth and slime buildup with varying degrees of success due to issues such as organic residues, slime buildup and incompatible pH. Chlorine and hypochlorite are readily sequestered by organics residues and slime materials, which protect active microbes from being killed. Hypochlorite also rapidly loses its efficacy above pH 7.5, below the natural pH of seawater and many ground water types with pH levels greater than 8.